The present invention relates to blow molding apparatus and, more particularly, to improvements in the way bottom plugs associated with such molds are actuated between their extended and retracted positions.
Blow molded plastic bottles are typically provided with recessed, generally dome-shaped bottoms to provide increased structural integrity. To produce such a dome in the finished bottle, a corresponding xe2x80x9cplugxe2x80x9d or xe2x80x9cpush-upxe2x80x9d is provided in the bottom of the blow mold cavity so that as the small parison stretches and enlarges during the blow cycle to meet the confines of the mold cavity, the bottom of the bottle partially forms around the protruding plug to conform to its shape and present the dome. However, to then facilitate discharge of the blown bottle from the machine, the bottom plug must be retracted out of the bottom of the bottle.
Typical injection blow mold machines have an upper plate-like die set that reciprocates vertically toward and away from a stationary, lower die set mounted on the machine bed. Cooperating blow mold halves are carried on the die sets to form blow mold cavities when the upper die set is in its lowered position, and to open such cavities when the upper die set is raised. A corresponding number of bottom plugs are arranged in side-by-side relationship and shifted horizontally into and out of the respective cavities for use in forming bottom domes in the blown bottles.
It is known in the art to actuate the bottom plugs using interacting cam wedges and return springs. Using this technique, a first cam wedge carried on the upper mold half comes into mating engagement with a second cam wedge carried on the bottom plug associated with the lower mold half. Thus, as the upper mold half moves downward into a closed condition with the lower mold half, the top wedge moves into engagement with the bottom wedge, camming the plug horizontally inwardly into proper position within the mold cavity. Then, as the upper mold half rises at the completion of the blow cycle, a return spring associated with the lower wedge pushes the lower wedge and associated plug back out to a retracted position so that the plug is withdrawn from the dome in the bottom of the blown bottle. Each mold cavity has its own set of cam wedges, its own set of return springs, and its own set of guide rods on which the bottom wedge reciprocates with its plug during actuation.
The conventional cam wedge actuating mechanism has many shortcomings. For one thing, the point of physical interaction and engagement between the wedge surfaces is vertically offset from the horizontal line of action of the plug itself so that the force of the interacting cam surfaces imparts a moment that tends to rock and cock the plug during actuation. This can lead to a multitude of problems, including failure of the plug to withdraw when the mold halves open because the bottom wedge is jammed on its guides and the return springs are simply incapable of providing enough force to break it loose. If a plug fails to withdraw, bottle quality can be compromised as the bottle is forced off the plug during the eject cycle.
The known cam wedge mechanism quickly becomes a problem due to premature wear of its various mechanical parts. It is important to grease the guides on which the lower cam wedge reciprocates, yet the grease has a way of working its way up into the bottle cavity areas, causing unsightly marks on the finished bottles. This can lead to rejecting entire production runs if even just a few rejects are noticed during random sampling. Furthermore, frequent tedious maintenance is required to replace prematurely worn springs, to clean the numerous components and to regrease the slide surfaces associated with the moving lower wedge.
The present invention provides a highly reliable, trouble-free actuating mechanism that supplies smooth, positive actuation of the bottom plug in both directions of actuation. No return springs of any kind are utilized. Cam surfaces interact with follower structure during both the extend and retract strokes of the bottom plug.
In one preferred embodiment, one or more cam assemblies are carried by the movable die set for reciprocation therewith during movement toward and away from the stationary die set. Each cam assembly includes an extend cam surface and a retract cam surface which are located in mutually spaced relationship on opposite sides of a follower carried by the bottom plug. Thus, as the movable mold half closes against the stationary mold half, the moving extend cam surface engages the follower and causes a reaction force transverse to the direction of die set travel which pushes the plug to its extended position within the mold cavity. Then, as the movable mold half opens at the completion of the blow cycle, the reversely moving retract cam surface engages the opposite side of the follower, causing a force in the opposite, transverse direction to withdraw the plug out to its extended position.
In a most preferred form of the invention, the mold halves open vertically and the upper die set is the part that reciprocates, the bottom plug having a horizontal stroke. Preferably, the cam surfaces are on separate cam rods operating within upright bores of a guide block that has a horizontal passage intersecting with the cam rod bores and slidably receiving an operating shank associated with the bottom plug. The shank has a slot that carries the cam follower, which is preferably in the nature of a roller, and the two cam rods are received within the slot on opposite sides of the roller to cause the shank to reciprocate horizontally within the passage as the cam rods are shifted up and down during raising and lowering of the upper die set.
In one preferred embodiment for multiple cavities, a series or gang of the bottom plugs are secured side-by-side on a common mounting bar for actuation in unison. Multiple gangs can be handled, with each gang secured to its own bar. Two or more of the operating shanks project rearwardly from each bar adjacent opposite ends thereof and are received within a pair of corresponding guide blocks mounted on the lower die set. Each guide block has its own cam rod assembly that reciprocates vertically therein during raising and lowering of the upper die set. Thus, all plugs of each gang of cavities are operated in unison for positive extension and retraction, using only a pair of cam assemblies and a pair of followers for the entire group instead of a pair of cam wedges for each cavity as in conventional constructions.
In another preferred embodiment each bottom plug is split into two halves which come together to present a complete plug when the two mold halves close. One plug half is carried on the movable die set while the other plug half remains with the stationary die set. Separate actuating mechanisms are provided for the separate plug halves, and the plug halves of each die set can be ganged together for actuation in unison.